$\begingroup$I wouldn't create a religous cult, or anything like that. Just let them set up a government like the US, make sure no-one knows how to operate the ship, and have a secret wealth of knowledge that would be available to everyone when they reach their destination.$\endgroup$
– Xandar The ZenonJan 19 '16 at 14:30

2

$\begingroup$Have you tried looking at it from this perspective: If a large enough part of your 20k population (so ~16-17k), by their own free will, decides to not bear children or otherwise act in an population unsustainable way that you can't do without strict matchmaking rules (which would be the case at around <4k according to some previous question on the site), then sustaining the population is likely not in your particular societies' best interest. Sustainability of their race may simply not be important enough to them compared to however they spend their lifes. That is also an option.$\endgroup$
– vruvreJan 19 '16 at 17:55

5

$\begingroup$I'm vaguely reminded of The Moon is a Harsh Mistress, which has multi-generational plural marriages. Also I'd think preventing people from reproducing would be a greater problem than encouraging it. Assuming that the ship wasn't built with copious spare capacity, you really don't want people reproducing faster than the rate of attrition. So think forced contraception and/or abortions moreso than forced breeding.$\endgroup$
– arothJan 20 '16 at 3:57

17 Answers
17

The problem of love and marriage falls into a major problem (DNA variety) and a bunch of minor problems (not so many fish in the sea). Surprisingly, the Major problem is far easier to fix than the minor, and it actually fixes a lot of the minor problems in the process.

Gene Banks

The main reason why love and marriage is such a problem on a multi-generational ship is that letting anyone procreate with whoever they want risks narrowing the gene-pool, and you can't have a big (living) Gene Pool because there's limited space on board. Well, what if the gene pool doesn't have to be alive? It would be far easier/smaller/more effective to keep a massive sperm/egg bank. Cryopreservation of sperm is cheap(er) in space — you'd need radiation shielding and some way of dealing with any close fly-by's of heat sources you have to do, but it would still be vastly cheaper and easier than on earth — and apparently frozen sperm has no expiration date.

All humans are sterilised at the earliest opportunity. In the case of females, all the eggs they will ever create are present before they're even born, so they can be... harvested (just to make this seem more evil than it is), and added to the bank. Then when two adults love each other very much and want a child, the computer determines the optimal pairing from the bank and bingo.

Why Would This Work?

Genetic diversity would be even easier — you wouldn't be limited to just the best genetic match between similarly aged members of the crew, you could choose the best possible combination of egg and sperm from a sample of the entire human race (or close enough, I'm imagining there would be a space limitation).

No requirement to have children — One of the big issues with the "must make the right people mate" method is that it means that certain people are forced into having children. If the DNA of the child is independent of the parents, there is no longer this requirement.

Parental love — because the child still comes from the womb, maternal/paternal instinct would be as strong as ever. You can teach that this is perfectly normal and it wouldn't even be strange if white parents "gave birth" to a black child, that's just the way it is. I think this is preferable to test tube babies (though that would be possible), because the family unit is still an important concept, and it will be needed at the other end of the journey.

Options

For those of you worried about sterilisation from birth in case of a bank accident that destroyed the gene stocks, clearly you would keep backup storage in different sites. In case of catastrophic loss of the entire system, I would argue that a tin can floating through a vacuum has bigger problems to worry about, but there have been recent advances in reversible vasectomies, including this novel approach which is literally a switch that can be turned on and off (though it takes a while to turn the tap off again after). If you're worried that this will be a temptation, let only the chief medical officer know about it, though the less secrets on a boat the better.

Going the technological route again, include a failsafe in the landing mechanism - if at any point the system in control of landing loses contact with the procreation systems for an extended period of time, it could trigger the reenabling of the reproductive system if the switches are fitted with some sort of RF trigger.

The Minor Problems

This ultimately boils down to "Not so many fish in the sea". 20,000 people is the size of a medium sized town. Given this small sample space, there will be a limited amount of people of a specific type of personality, so unless genetics and personality prediction comes on massively before this ship sets off, it is likely that every now and then there will be a mismatch between the amount of people attracted to a specific personality and the amount of people currently on board with that personality.

This I fear is unavoidable, and dealing with heartbreak and loss is where the family unit will come in. There are of course a number of other aspects which are solved by the Gene Bank approach:

Free love — Anyone can love anyone they want, even their sibling they want to. All the social taboos that arose out of the need for genetic diversity are no longer valid.

Non-heterosexuality is fine — because noone is required to have children, all forms of sexuality are fine, so long as enough people are still happy to parent children. Make test tube babies (The spaceship form of Adoption) socially acceptable, and it makes that even easier.

Racism basically disappears — because you have no idea what your child is going to be like, and because families can be made up of radically different genes, it destroys the segregation that allows racism to form.

You would still get sex-related crime due to jealousy, probably, but this would allow for a much more liberal society than otherwise. Through sterilisation you have made it physically impossible for anyone to break the rules, but have provided a system whereby everyone still gets to experience everything you go through having a child. Yes, you can argue that people would like to raise a child of their own genes, but that's your society talking. It's not something that is strong enough to rebel over (and I'm saying that as a parent). It's a far far easier mentality to change than arranged marriages!

Ultimately, it's not even one you have to lie about. When people learn about biology and DNA, they can be told about why the process is necessary. They can be told about the Gene pool, how we all share genes and how "There's a little bit of all of us in everyone else on board". This will help engender an overall feeling of community.

If I haven't considered anything, let me know and I'll see whether I can fit it in!

$\begingroup$I was going to suggest this. Just ship a lot of sperm and eggs with you. Provided the average birth rate is high enough to maintain the necessary crew, you can sort the genetic diversity issues out when you get there.$\endgroup$
– superluminaryJan 21 '16 at 12:27

$\begingroup$Comments are not for extended discussion; this conversation has been moved to chat.$\endgroup$
– VincentJan 21 '16 at 17:05

1

$\begingroup$Genetic material in space should be heavily radiation shielded, so it's non-trivial to do that and also radiate heat away into space. Thermal equilibrium with background radiation in space is something like 3 Kelvins, but the key point is that vacuum does not conduct heat. Any internally-generated heat in the ship will make its temperature at thermal equilibrium much higher than that. It's still probably easier to keep some samples cold on a ship in space than in a lab on earth, though, esp. if you're concerned about moving parts / failsafes.$\endgroup$
– Peter CordesJan 22 '16 at 12:40

1

$\begingroup$@PeterCordes, Yup, that's "some way" of doing it. I was going by This, which implies that you would have to deal with a significantly different heat gradient if you went close enough to a star. I figured a ship large enough to hold 20000 people was large enough to have a section which could be effectively isolated from the heat-generating equipment without much difficulty, so I wasn't worrying about that as a primary source of heat.$\endgroup$
– Ieuan StanleyJan 22 '16 at 13:10

1

$\begingroup$@IStanley: you have to fly very close to a star (in interstellar terms) for heating to be a factor. If you blitz through a solar system, you're in much more danger of colliding with debris than if you stay out in interstellar space. Since you're in a generation ship, it's probably sub-light moving at relativistic speeds, with relatively weak engines. This means it's nearly impossible to steer, so your only hope for collision avoidance is to stay way the hell away from things. I mean sure, your cooling system needs to be able to handle being near stars, but only at the start/end.$\endgroup$
– Peter CordesJan 22 '16 at 13:38

We built multi generation ship which can get 20 000 people to another solar system in next 20 generations (600 years)

Good news! You've got significantly1 more than a minimum viable population! What does this mean? You have enough people that genetic variety will not be a significant issue unless something kills off more than half of your population.

What constraints do we still have? Obviously, the ship isn't going to get any bigger, so there's a hard limit to how big the population can get. Also, you don't want the population to drop, so you've got to replace everyone that dies.

Something else to consider is complexity—the more complex the system, the more likely it will be to degenerate, especially over the course of 600 years. You shouldn't rely on computers to manage the complexity for you because there's always the chance that a hard disk will fail and lose important information you need. So what you need is something simple but effective that people can keep track of.

The plan:

The standard expectation is heterosexual monogamy—people are allowed to marry who they want to and have two children together. It is also expected that there will sometimes be non-standard situations. The most common will be couples who cannot have children, including both homosexual couples and infertile couples. Whenever that happens, a deficit of children will be noted, and couples can apply to have an extra child to fill that deficit. As such, it will be possible for a couple to have three (and rarely four) children. Multiple births (twins, triplets, etc.) can have the opposite effect—a surplus will be noted, and the next time a deficit is noted it will be automatically cancelled out by the existing surplus.

The ship should be kept at 90–95% capacity to allow for surplus supplies to be collected and to handle temporary surpluses in children.

This child count can also be used as a tool to encourage fidelity and family stability. In the case of divorce, the child count will be split between the two parents. If they get divorced before having children, there is no penalty. If they have one or two children, then each adult has a child count of one half or one and in a subsequent marriage to a previously-unmarried person they would only be able to have one additional child. In the case of a divorce with cause (abusive spouse, for example), the at-fault spouse could receive the full child count while the other spouse gets custody. If this occurs with two children, then the at-fault spouse incurs a child surplus.

It is also possible to have an imbalance of men and women. A surplus of women is a minor problem—polygyny (one man having multiple wives) can solve this without causing any confusion about parentage of children. This would only be permitted when there is a surplus of women, and the surplus women (wow, that sounds terrible. They'd have a much more polite term) might be encouraged to join childless couples. In these cases, the families would be permitted a number of children equal to the number of adults. A surplus of men is a bigger problem. Letting a woman have two husbands makes it hard to tell who the father of a child is.

Unfortunately, the natural ratio of men to women is a little unbalanced in favor of additional men (somewhere around 1.05:1). This means that minor measures should be taken to counter this. For example, I'd suggest that dangerous tasks should be assigned to men.

With these standards, it should be pretty easy for everyone to keep track of their genealogy to at least their great-grandparents. This makes it easy to make marriage to close-relations taboo; marriage between siblings or cousins will be forbidden, and marriage between second cousins would be discouraged (though not forbidden).

1. A minimum viable population is somewhere around four thousand individuals. That is level at which a wild population can survive without human intervention. In other words, without planning. As long as you're above that level, you're good to go. If you fall below that level, you'll need to start taking measures to encourage genetic diversity. With a comprehension breeding program, you could survive falling down to a couple hundred people.

$\begingroup$+1 for neatly solving the problem without some wacko society that destroys the entire motivation to go$\endgroup$
– DCShannonJan 20 '16 at 3:57

$\begingroup$+1 for recognising the minimum viable population thing and all the rest :)$\endgroup$
– BounceJan 20 '16 at 8:59

$\begingroup$+1 It's nice, and simple like you said. It's not foolproof though - there is no safeguard from a natural loss of genetic variety. Say a particular genetic line ends up having a lot of people in dangerous jobs and couples who don't have children for a variety of reasons. It would be very easy to accidentally lose large portions of the gene pool on such a manner. Are you going to force heterosexual couples to have children? How would you deal with certain genetic lines being predisposed towards multiple births (twins run in the family), potentially destabilising the genetic balance?$\endgroup$
– Ieuan StanleyJan 20 '16 at 22:24

$\begingroup$@IStanley well, nothing's going to be absolutely foolproof. Anyway, with 20000 people you have a lot of spare genetic variety. You would have to lose more than 3/4 of your genetic variety before you reach the area where variety actually starts to be a concern.$\endgroup$
– Rob WattsJan 20 '16 at 22:54

$\begingroup$If a hard-drive failure can lose critical mission data, I would not sign up for this mission. Holy crap! We already have software that can successfully keep n redundant copies of critical data distributed across a set of storage servers. It's cool that you've come up with a scheme that humans can take over if things go completely to hell with the technology, though. (perhaps after arriving, but still depending on limited resources, and then some disaster happening to the ship). But losing critical data needs to be extremely unlikely if your ship is going to keep working.$\endgroup$
– Peter CordesJan 22 '16 at 12:48

Trying to engineer the culture of a generation ship (or a space colony or even an Earthy city or university campus) could backfire wildly if it is too far from the "norms" of the founding society. An ship crewed by Indians or Chinese will not work too well if they are expected to follow Western cultural norms, nor should you expect a ship with a largely American contingent to be receptive to living under a Confucian social structure in a Chinese ship.

So the social and cultural norms of the launching society will most likely be in effect on board. Since one of the issues to address is genetic variability, some current social norms will carry over well (most Western cultures discourage marriage between first cousins, for example), and Americans, with their notions of physical and social mobility, will probably keep the DNA pool well stirred without much further prompting.

Establishing a religious or quasi religious cult to ensure the goals of the ships launch crew are maintained over time is more problematic. It is possible to create extremely static cultures (Ancient Egyptian culture lasted for 3000 years with change happening at glacial speed), but the people raised in these cultures will probably lack initiative and flexibility to deal with shipboard emergencies or the changing conditions once they reach the target star.

Indeed it seems almost impossible to avoid social, cultural or even linguistic drift, especially over extended periods of time. Shakespearian English was spoken only 500 years ago, yet is virtually incomprehensible to the modern English speaker, to give an example, and you can also look at the social and political structures of late Tudor England, or the sorts of sports and games popular then compared to now to see just how much change is really possible in that time.

The best way to ensure transmission of ideas over the long term might be to "encode" them into legends and children's stories. People learn things on their mother's knee which can stay with them for life, and be transmitted to the next generation. Sometimes the meaning is lost over the years ("Ring around the Rosy" is a song about the Black Death, for example), and sometimes the culture refuses to let go of the past (when I was deployed in former Yugoslavia, the Serbians told us stories of the Battle of Kosovo Polje in such vivid detail that it could have taken place in the 1990's. It happened in 1389!), so this is a plan with a mixed chance of success.

The only real way to ensure the passage of the founder's intent is to have the founders themselves arrive; either the ship is a giant freezer carrying frozen colonists, or the colonists are carried in vitro and "birthed" on arrival to be taught the Founder's intentions, of the ship moves at close to the speed of light, so very little time passes on board compared to the outside frame of reference.

I don't think there is a problem. Through most of mankind's history we lived in much smaller communities. The typical hunter/gatherer tribe is 150 people, and they may meet with up to 10 or 20 other tribes to exchange marriage partners giving a total community size of no more than 3000. In fact until the 19 century 20000 people would be a large city. So let people marry as they please.

If you're still worried about genetic variety you could have the sperm/ova bank mentioned earlier, and encourage each couple to have one child from there. That should give plenty of genetic variety without getting heavy handed about it.

I think the best solution is probably to detach "relationships", "procreation", "child-bearing", and "child-rearing" as much as possible. Lois McMaster Bujold's Vorkosigan novels have the technology of artificial wombs, which allows the work of carrying children to term to be delegated to machines. This frees women from an obligation while improving safety as well. It would be possible to mandate that everyone donate sperm and eggs — genetic diversity is a valuable communal resource — while not actually forcing anyone to do the particular work of child-rearing. There's no need to impose restrictions on anyone's sexuality then.

It's also important to think of how the colonists would be aware both of the mission, and of the environment of the colony ship. Everyone on board would be an extreme environmentalist by Earth standards: every resource is finite and limited, nothing can be thrown away, any damage to the spaceship is a threat to all aboard, and the crew know that it is their responsibility to look after it for future generations.

In that kind of environment, some form of commune structure would probably thrive. Childrearing would be shared. "It takes a spaceship to raise a child". Relationships among the adults would be entirely flexible so long as they weren't harmful to the social order. So Mormon-style polygamy would not be allowed but Heinlein-style probably would.

$\begingroup$Yes, absolutely, that's what I was trying to convey in the first sentence. Children are already being brought up successfully by various combinations of one, two, three or more parents of various genders and sexualities.$\endgroup$
– pjc50Jan 20 '16 at 13:25

$\begingroup$Ah... it wasn't completely clear that's what you meant. I read it more like "this needs to be done" rather than "this is already the case, people just don't realize it."$\endgroup$
– T.J.L.Jan 20 '16 at 13:26

Just enforce a maternal equivalent of the draft to your female passengers. At the onset of their physical maturity, all male passengers will contribute to a sperm bank; and through artificial insemination, all newly mature female passengers will make a withdrawal. A computer will play match maker so that genetic diversity can be maximized.

After that first child is born, their mother is free to have additional children if they choose, conceiving them either artificially or naturally. Bisexual and/or mono-sexual dating, co-habitation and marriage would all continue at the participants' discretion; and all eligible females would be single-mothers, so there would be no social stigma involved.

To balance the tables, all male passengers would be required to spend nine months doing community service in the human-waste processing plant, and/or outside the ship's hull. Also, all jobs involving exposure to radiation would be reserved to male passengers who have already contributed to the bank.

Because it's a relatively small group of people, you'll want to have some system to rule out accidental incest. Iceland is currently doing that now, and they have over 300,000 people.

You may want to get rid of love as an aspect in marriage and family planning, going back to Victorian-style cultures. Remember, that is a relatively new development... Romeo & Juliet was originally written as a satire of the terrible things that happen if you follow your heart rather than marrying who your family wants you to marry (Spoiler: you're going to die) and only recently (last 80 years) have we started reading it as "look at this tale of love breaking down walls" which was not the original point. If this is reinforced strictly on the first several generations, this system WILL break down over the 600 year period. You'll want to build whatever anti-incest system SEPARATE from the rigid family/marriage structure so that will remain independent and required, while the social norms change over time.

$\begingroup$...to rule out accidental incest. Why? That is, if there are particular genetic flaws that are recessive and normally not expressed, and the incest stays constant and pervasive over enough generations somehow to ensure expression across most of the population, then it would possibly be meaningful. But otherwise, what's the problem? Surely legal property or privilege inheritance won't be complicated. So what else is there?$\endgroup$
– user2338816Jan 19 '16 at 23:19

2

$\begingroup$Because you want a diversified genome. As I said, that's already a problem in modern-day Iceland with 15x the number of people... when you're starting with only 20k people, you want that genome bank to be diverse as possible for as long as possible. Regardless, incest results in "spontaneous abortions of zygotes, perinatal deaths, and postnatal offspring with birth defects", in addition to being more vulnerable to similar diseases. If you want more science, look up the references on the bottom of the Wikipedia page for incest.$\endgroup$
– VogieJan 20 '16 at 14:16

1

$\begingroup$Comment was about accidental incest that can imply infrequent and irregular rather than continued over generations within a small sample. A diversified genome will be virtually unchanged since the same genes will be passed from each parent. Population is easily large enough. Further it'd be unsane not to pre-screen population to minimize genetic abnormalities and maximize desired traits. Some limited incest could be beneficial. I wouldn't have commented if I was unfamiliar with the documentation, Wikipedia or elsewhere.$\endgroup$
– user2338816Jan 21 '16 at 1:16

$\begingroup$More generally, genetic screening can make sure an embryo doesn't get a double-recessive nasty, no matter how closely or distantly related the parents are. Those rules are just playing the odds: what's the point of that if you can know for sure?$\endgroup$
– JDługoszJan 22 '16 at 15:16

This issue has already been explored in surprising detail in Robert A. Heinlein's The Moon Is A Harsh Mistress (which is excellent in a lot of dimensions). In this case, the "ship" is actually the moon, but it's probably worth reading the story so that you can see what has already been explored in this idea space. It is also touched upon in Neal Stephenson's Seveneves (which I am still reading...also great so far). Stephenson clearly did his homework, so I think it's worth reading for the science (whereas, Heinlein focuses on the sociological aspects).

If you only make small changes to current society, people might want to fall back to different patterns that they remember or read about somewhere. Establishing a social system that stays stable for 20 generations in a row is rare.

You can get rid of that if you simply get rid of the males, which mandates a drastically different system. Use sperm banks, pre-filtered for X chromosome only. Establish pregnancy at specific ages like we have school at specific ages in today's societies. Some current societies have one or more years mandatory military service at around 20, or mandate working in a factory for several months before university. There's some way to bend the rules in all these cases so the same should apply to pregnancies — just make it so there's an expectation to get pregnant at a certain age, and some economic drawbacks if you refuse.

If you only have females they only need to have one child on average instead of 2 (plus/minus early deaths, twins, women who really want a second pregnancy, women who are infertile, women who want to opt out, etc), which also might make things easier.

This setup allows the population to make lots of changes on their own over the generations, but self preservation mandates that certain constraints are upheld, such as population control.

But honestly, for a 600 year journey I'd use several smaller ships with different social systems on each, and hope no more than half of them die from technical failures. Only 100 fertile women and a well stocked sperm bank are enough to make a viable population, so even a single ship surviving will be good enough.

Love and marriage would continue to function based on your pre-ship cultural norms, but the children you raise might not be yours.

Psychological Health

Psychological health is really important when you’re in a metal tub flying through space. It’s so important that deep psychological studies are being done merely on the approximately 500 day round trip to Mars. A generation ship is an entirely different level of magnitude, which ups the stakes quite a bit. You really don’t want the first generation to mutiny or dissolve fifty years in and you definitely don’t want a random late generation to do it. Love is a very fundamental human emotion and one on which quite a bit of chaos can be sewn when not respected. As a result, you really do not want to interfere with a person’s ability to love or marry whomever they choose.

The core problem that raises this question is genetic diversity. In such a small population over a long period of time you want to truly maximize it, and it’s clear that traditional means of free marriage are not constructed to support that. However, there is a solution: artificial insemination or in vitro fertilization.

Carefully Planned Children

Birth control would be heavily encouraged if not mandatory for couples. A genetic database would allow couples to match based on the desired genetic diversity, but this would not be required. Births and pregnancies would be heavily planned with both limits and quotas (with special exemptions as necessary). If you're a couple that is properly genetically diverse, you may be allowed the privilege of having children naturally. For everyone else, the woman would be required to have some percentage (if not one hundred percent) of her children with the genetic material of someone else on board. The man would, likewise, be required to contribute his elsewhere on the ship. In spite of this, the couple would be expected to raise the resulting children as their own.

Accidental Pregnancies

These would happen, but hopefully infrequently enough that they could be handled on a case by case basis with the child coming to term. It’s possible that these children would be excluded from the collective pool of genetic material in the future.

I think a 'religious cult' would be a dangerous way to go. Not the least of which you would have to start 'indoctrinating' the passengers before you even put them on the ship. Religions change and mutate, and even more they fracture and split.

When you put in place a religion to 'keep people happy and focused on a goal' you have built in a flaw that could jeopardize the whole mission. While religion beliefs have a purpose, creating the mission to be the purpose will eventually cause distension on the best way to fulfill that purpose. And once you put faith behind it, then decisions will become matters of doctrine and purity of purpose, not what might be the safest or best way to accomplish the goals. Or even if the original goals turn out to be impossible, what are reasonable alternatives? Using religion in the way you are suggesting will quite likely cripple scientific decisions that make sense at the time.

With around 20,000 people most people would be able to do what they want, within reason. I think there would be some gene tests so that there are people that 'shouldn't' be paired (like you immediate family) only it would be a little wider spread.

Why prevent homosexual relationships? They have been around forever and will pop up even if there aren't any in the beginning. If they want kids they can have kids, if not don't. Since there will be population limits as to how many can live on the ship at any one time, who can have children and when will need to be controlled.

Someone of course would have to monitor this, to make sure that an even distribution of genes continues and mixes. But if a homosexual woman wants children the genes could come from a man not interested in raising any.

The point is, while someone needs to keep good records (and using paternal DNA for each child) there should be enough mixing with 20,000 people that there shouldn't need to be much direction from above. Having kids might become more of an artificial insemination process, you can love who ever you like, but your children are from the community and for the community so it's your responsibility to have the best one for everyone (meaning it might not even have your dna)

Your population size seems acceptable. I've read somewhere (can't find the reference now though) that the genetic variability between relatives that are removed further than first cousins is not more problematic than in the general population. (Eurocentric, but I suppose that the specific population could be taken into account, since it is said (again, no reference) that there may be more genetic variation in a single African village than across the whole of Europe.) Cousin marriages have been quite common in history in Europe and the Middle East, and still seem to be in certain parts of the world.

Since your ship's population is the size of a medium sized town, I am for interest's sake suggesting a different approach to most of the other posters: instead of inventing all sorts of sci-fi interventions, why not go the opposite direction and organise a conservative society similar to an era when a town of that size would be the norm? That would be the starting parameters for the environment (meaning the darwinistic term), but it probably would adapt (through natural selection) over the course of its journey. (When arriving at the destination, the environment would of course change again, where a new course of adaption would be needed.)

One problem that you mention is the whole love/offspring thing. A slight adaption to current western culture, just moving a little bit back in history, might be to have parents arrange marriages. While this has a negative connotation in modern western culture, it does not need to have, if parents have the well being of both their offspring as well as their society at heart. It is not too long ago that this was acceptable even in some western cultures, and history has shown that the modern invention of marrying due to some not-quite-mature adult's youthful feeling of infatuation does not necessarily result in better choices, more stable families and thus more a stable society. (A computer could provide input as to the expected genetic desirability of the offspring, but I think this is not really necessary.) In any case, a society with such "conservative" norms may help in keeping the gene pool healthy (if the norm is to have children, and people that may otherwise be inept at obtaining a mate or too selfish to find one, are "nudged" into the right direction.)

The other problem would be to keep the population size as big as allowed by resources (to ensure maximum variability) but no bigger. This would probably by regulated by the already well-known "procreation license" system. I see that this will create, over time, an "offspring economy", where the permission to procreate may be linked to money (or some other value), either negative or positive (because it also costs resources to rear that child). Then again, it seems that one of the ship society's main purposes would be provide enough resources so that each child may reach adulthood, so the cost of childrearing may be taken away from the individual and become society's problem in a very socialist way. It needs to be pointed out that from a genetic PoV (and maybe others) one child would not be exactly equivalent in value to another: if variability is of high importance, a person's (genetic) value decreases with each sibling he/she gets.

I would argue that it would be most desirable to maintain the largest amount of genetic variability on ship until it reaches it's destination, where the most desirable traits for that environment are then to be allowed to be selected by natural selection. The on-ship population should not be allowed to homogenise. If people are from different races, it may be a good idea to keep it that way. There should be high-IQ and low-IQ people. Technical people, artists, dreamers, warriors, acrobats, thinkers, talkers, listeners ...

The mechanics are of course all interesting, but what's even more interesting in my mind is the human factor behind it. You could have the same mechanics, and have a very authoritarian or a very liberal society, a society where off-norm is very negatively "frowned upon" or one where the norm is encouraged in a very loving way.... and all might have vastly different outcomes. Since social aspects are as much environment as the tech is.

It's of course a thought experiment - as worldbuilding should be - but I guess if one writes a new scifi novel, going contrary to the beaten path might bring some interesting results.

$\begingroup$"that there may be more genetic variation in a single African village than across the whole of Europe." - Easily disproved if you consider that Europe has African immigrants.$\endgroup$
– PeterJan 22 '16 at 14:26

20 000 people is easily enough genetic diversity. Just make it so that people are not allowed to marry cousins, which I believe is already the case. So you don't have to do anything special to take care of inbreeding. The main issue is keeping up the size of the population, you could introduce laws where people are not allowed to have more than 3 children, and if there aren't enough children then introduce incentives, like better food or living quarters for people who have children

$\begingroup$Occasional cousin marriages in a population that size won't actually cause any harm. What is wrong with cousin marriages and why they are illegal in some places has to do with inheritance of property and the incentive this creates to repeatedly marry cousins over many generations. That is genetically damaging.$\endgroup$
– nigel222Jan 30 '16 at 12:56

People that are already married are unlikely to wish to join the ship unless their partners are also allowed to join.

People with world views that don’t much how the ship is going to operate are also unlikely to join. This is very important, that if you wish to have a good mix of genes you need people from different countries to join. Therefore is it possible to have a single system of “partner choose”?

I assuming you are choosing the 20,000 people base on the skills they have, therefore they will be mostly of “post education” age maybe a lot older. But…

You need to have a mix of ages joining the ship; otherwise they will all become old within a few years of each other. There could also be issue with them all having children within a few years of each other. So I expect that you will need some people that are over the child baring age, or to set out with a lot less people then you have space for.
I also expect you need children to be joining the ship with their parents, so that you don’t have a gap of 18 years before the first set of new “trainees” start work.

You don’t want people in the same job beading with each other too much, otherwise you will get lots of autism with “thinkers” breading with each other. Therefore the method of choosing partners must not just be based on who someone works with.

Given partners will want to be of about the same age, and you will have an age profile from 0 to about 80 once you have been going for a generation. Assuming 3 years between partner ages, there are only 20,000÷80 × 3 ÷ 2 =372 people that each person can choose between as their partners. So it is very possible to have a system when someone meats with every possible partner.

Everyone would have gone to the same school as their partner!

As it is already clear that peoples parents will be of different races and hence have different “world view”, the next generation will have a mix of world view from the parents. This will reduce the choose of partners evens more, as someone is most likely to choose a partner with their own worldview.

As others have noted, the problem is more likely to be too many children than not enough. I suspect a generation ship would have to put limits on how many children couples could have rather than trying to force them to have more children. I suppose this could be a problem if the initial crew was made up entirely of infertile people, hermits, and homosexuals. Or more realistically, if there were enough such people in any given generation along the way that population fell. But unless something about the nature of life on the ship makes people not want to have children or incapable of having children, there would likely be enough people who want to have ten children to make up for those who have none.

Trying to create unusual cultural norms for such a ship faces one tough problem: Presumably once the ship has been on its way for a couple of months, it has traveled beyond the reach of anyone back on Earth. If you had ships that could flit back and forth across the light years in a few hours or days, you wouldn't need a generation ship, you'd just send people in these FTL ships. So you can make up all the rules you like for how these people are supposed to behave, but once they're beyond your reach, they can freely chuck the rules and do whatever they want, and there's absolutely nothing you can do about it. Even if you picked the initial crew to be people who agree to this unusual society that you think is a good idea, the next generation is going to rebel against it, or at least question it. That's what teenagers do. The idea that you are going to raise children in a carefully controlled environment so that they completely, slavishly follow everything their parents teach them is a pipe dream. People have been trying to do that for thousands of years, and it's never worked. (Every now and then I hear people who have just had their first child talk about how they are going to raise this child to follow in their footsteps and uphold all their best ideals — whether that means being tolerant and open-minded or working tirelessly to stamp out all the inferior races or whatever — and I can only smile and say, yeah, good luck to you. That's not how it works.)

Maybe, possibly, you could indoctrinate the original crew with brainwashing techniques and drugs and high-tech mind control, and then they do the same to their children, generation after generation. But would a crew of such doped-up robots really be the kind of elite you need to successfully complete such a dangerous and complex mission? And even if it worked, it would take only one generation, anywhere in the 600 year voyage, to say, No, we're not going to do this to our children, and the whole plan falls apart. (There might be a science fiction story there, about a few who decide not to brainwash their children, and now there are two factions on the ship, the brainwashed and the not-brainwashed, who struggle for control.)

If your ship was really big enough for 20,000 people, I don't think there'd be a lot of trouble with maintaining genetic diversity and allowing people to choose mates in the traditional way. If the crew was much smaller, if it was only a few hundred, I can see there being an issue that if, say, Al and Betty are the only people on board with a certain gene, that if they marry each other you risk losing that gene, and so they must be forced to marry other people.

The smaller the crew the more you'd have to work to insure initial genetic diversity. You don't want everyone on board to be, say, white Anglo-Saxons descended from people who lived in or near London. You want to make sure you have some whites, some blacks, some Hispanics, some Asians, some tall people, some short people, some artistic people, some detail-oriented people, some big-picture people, etc. A society advanced enough to launch such a ship is probably advanced enough to be able to do genetic tests to be sure that there is sufficient genetic diversity. If the "first draft" crew, perhaps selected for specific job skills, is missing some genes, that they could say, Hey, we need some people with gene A17G4, which controls predisposition to growing bananas, or whatever.

20000 people is a huge population on a generation ship.
With careful breeding 200 people have enough genetic diversity to support 80 generations (~2000 years) without running into problems.

With 20000 people, you'll have no problems, so I wouldn't worry about it.
As I Stanley said, you could also include more genetic samples in the form of sperm easily enough. Not eggs though, as I believe those have a frozen shelf life of 20 years with current technology.

Your biggest problem is space and resources. While population will fluctuate, the ship won't be getting any bigger, as there aren't really any resources to add on with in interstellar space.
You'd probably want a 2 child limit, with additional child bearing privileges given out through a lottery or something to make up for accidental losses over time and to make up for people that can't/wont have children.
A population spike would be a bad thing unless you're anticipating losses in the near future.

I wouldn't bother with a cult, as that could lead to other problems.
You could get the result you want by just drilling the rules into everyone, and starting young with new generations, so that when they are of age the idea of only being allowed 2 kids won't be questioned.

Another benefit of the birth lottery is so that people have hope. That way it's not the leaders of the ship telling you that you can't have that baby, its just luck not going your way this time. Maybe next time though!

Another thought is that there will be people that just don't want kids.
These people might not make the best parents if they are forced, so allow them to give/sell their privilege away to someone that really wants it.

I adore these thoughts and images, thank you all. To begin,we will need the first set of parents to be the healthiest, and the most straight, and all 'good scouts' type of people you can get. This is to give the next two generations the very best chance of being healthy. Then, when these first parents have kids, the recessive genes, and just chance, will start showing up, and you will start to see certain conditions, and every different type of human you can possibly get. To prepare for this, you also need some old people. Not just for their experience, but when they get sick, and die, the doctors on the ship will be able learn, and observe, and experience, what to do with illness, and what to do when a patient dies. Imagine if you filled the ship with 100 'perfect' couples.... 40 years later you will get a sick child, and the doctors won't have a clue of what to do. Each ship will need enough people, ages 45-70, getting on board, so there will be a good flow of sicknesses and death. (And maybe they can teach the youngsters on board some other ideas, along the way.) --- And as for quantity? I think the optimum number for a ship would be 3000-9000 people, with space to grow to 5000-12000 people. Instill a fear of sibling and first cousin marriage, and then after that just let people choose their own spouses. ( And there was a great idea earlier, not just one ship, but a dozen ships.) And the idea of needing to decide what to do about consanguinity? Or people choosing not to breed? Or homosexuality? Forget about it, once they launch, each ship is on it's own, and should be able to decide how they will handle with social differences.